Abstract
The continuous variations in crack shape or aspect ratio, a/c (a is the crack depth and c is the halfsurface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue crack growth of small cracks in a titanium (Ti-8Al) alloy. The significance of the aspect ratio variations in explaining the “anomalous” small-crack behavior was evaluated. The aspect ratio data were determined from the measurements of crack compliance, made using a laser interferometric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the order of a few grain diameters. The experimental a/c data were compared with the patterns of crack aspect ratio variation, obtained from the oretical simulations. The simulated data were generated by assuming alternate crack propagation at the surface and at the depth, the details of which are presented in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally observed variations. After incorporating the a/c variations in ΔK calculations, the scatter in the growth data of small cracks was significantly reduced and was found to be of the same order as in large cracks. Additionally, it is shown in this study that the conventional methods of analysis of small-crack data, performed with an assumption of a/c=1, can result in significant errors in ΔK calculation and an increased level of scatter in small-crack growth data. Small cracks also were found to exhibit low closure levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks, often referred to as anomalous, are due to the assumption of a/c=1 in situations of large variations in aspect ratio, the use of conventional methods of data analysis, and the lower levels of crack closure found naturally in small cracks.
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